Formation evaluation in petroleum source rocks (commonly referred to as shales) involves estimating petrophysical parameters of the organic matter in the rock formations, such as its thermal maturity and density. The organic matter here refers to the dispersed, solid, and insoluble organic matter in sedimentary rocks termed kerogen. Thermal maturity is important for evaluating reservoir quality, hydrocarbon quality, and hydrocarbon type. The density (specifically grain or skeletal density) of kerogen is important for estimating porosity. The measurement of kerogen properties (e.g., thermal maturity and density) as a function of depth is desirable in nearly every well drilled in a petroleum source rock.
In general, kerogen properties are determined from time-consuming and expensive laboratory techniques. For example, thermal maturity of kerogen has been estimated using vitrinite reflectance, in which the optical reflectance of vitrinite macerals in kerogen immersed under oil is estimated and expressed as vitrinite reflectance maturity (see Hackley, P. et al., Standardization of reflectance measurements in dispersed organic matter: Results of an exercise to improve interlaboratory agreement. Marine and Petroleum Geology, 59, 22-34 (2015).) An alternative laboratory technique for estimating thermal maturity is using programmed pyrolysis (see Behar, F. et al., Rock-Eval 6 Technology: Performances and Developments. Oil & Gas Science and Technology—Reviews I.F.P., 56(2), 111-134 (2001)) by measuring the temperature at which maximum decomposition of kerogen (Tmax) occurs and then calculating vitrinite reflectance from known correlations between Tmax and vitrinite reflectance. More recently, attempts have been made to correlate the thermal maturity to vibrational modes obtained by infrared (IR) spectroscopy. For the purposes of describing the invention herein, thermal maturity is quantified in terms of vitrinite reflectance units, % Ro, which is the scale upon which vitrinite reflectance measurements are quantified. Other scales for thermal maturity are known to those skilled in the art. With respect to kerogen density, determinations are typically made using gas pycnometry techniques known to those of ordinary skill in the art. The measurements are made on kerogen isolated from the bulk formation sample, which requires hazardous laboratory treatment of the sample with series of concentrated acids such as HCl, HF, and sometimes CrCl2, to dissolve inorganic minerals including silicates, aluminosilicates, carbonates, and metal sulfides, among others, and yielding a kerogen concentrate free of inorganic phases.
The infrared (IR) spectrum of kerogen varies as a function of its composition and structure. IR spectroscopy measurements respond directly to the type and abundance of molecular bonds, e.g., structure, in the material being studied. Therefore, IR spectroscopy may provide information on certain kerogen properties. Several structural indices for kerogen have been defined on the basis of IR spectroscopy measurements and several of these have been correlated to thermal maturity (see Chen, Y., et al., Characterization of chemical functional groups in macerals across different coal ranks via micro-FTIR spectroscopy. International Journal of Coal Geology 104, 22-33 (2012); Craddock, P. R., et al., Evolution of kerogen and bitumen during thermal maturation by semi-open pyrolysis investigated by infrared spectroscopy. Energy & Fuels 29, 2197-2210 (2015); Ganz, H., et al., Application of infrared spectroscopy to the classification of kerogen-types and the evaluation of source rock and oil shale potentials. Fuel 66, 708-711 (1987); Guo, Y., et al., Micro-FTIR spectroscopy of liptinite macerals in coal. International Journal of Coal Geology 36, 259-275 (1998); Ibarra, J. V., et al., FTIR study of the evolution of coal structure during the coalification process. Organic Geochemistry 24, 725-735 (1996); Iglesias, M., et al., FTIR study of pure vitrains and associated coals. Energy & Fuels 9, 458-466 (1995); Lin, R., et al., Studying individual macerals using IR microspectroscopy, and implications on oil versus gas/condensate proneness and “low-rank” generation. Organic Geochemistry 20, 697-706 (1993); Lis, G. P., et al., FTIR absorption indices for thermal maturity in comparison with vitrinite reflectance Ro in type-II kerogen from Devonian black shales. Organic Geochemistry 36, 1533-1552 (2005); Painter, P. C., et al., Concerning the application of FTIR to the study of coal: A critical assessment of band assignments and the application of spectral analysis programs. Applied Spectroscopy 35, 475-485 (1981); Tissot, B., et al., Geochemical study of the Uinta Basin: formation of petroleum from the Green River formation. Geochimica et Cosmochimica Acta 42, 1469-1485 (1978).)
Structural indices for estimating thermal maturity have been developed by quantifying one or more of the following IR absorption bands: aromatic CH out-of-plane deformation (about 700-900 cm−1), aliphatic CH3 symmetric deformation (about 1375 cm−1), aliphatic CH2 symmetric deformation (about 1450 cm−1), aliphatic CH3 antisymmetric deformation (about 1460 cm−1), aromatic C═C stretches (about 1600 cm−1), oxygenated (carboxyl and carbonyl) stretches (about 1650-1770 cm−1), aliphatic CH2 and CH3 symmetric and antisymmetric stretches (about 2800-3000 cm−1), and aromatic CH stretches (about 3000-3100 cm−1).
Most of the structural indices derived to date are limited to the measurement of kerogen isolated from the surrounding rock (mineral) matrix, because most organic IR absorption bands (those below 1800 cm−1) are otherwise obscured by more intense IR absorption bands associated with inorganic minerals. IR absorption bands associated with kerogen between about 2800 and about 3100 cm−1 are readily amenable to study in bulk samples. Therefore, art based on the IR analysis of isolated kerogens is not necessarily useful or applicable to rapid measurement of bulk formation samples.
Methods exist to estimate the thermal maturity of kerogen in bulk formation samples using IR spectroscopy, for example, as described in U.S. Pat. No. 8,906,690, which is hereby incorporated by reference in its entirety. These methods are based on spectral deconvolution and curve fitting of measured IR spectral features between 2800 and 3000 cm−1 related to absorption bands of the following vibrational modes: (i) a CH2 symmetric stretch centered at about 2849 cm−1, (ii) a CH3 symmetric stretch centered at about 2864 cm−1, (iii) a CH stretch centered at about 2891 cm−1, (iv) a CH2 antisymmetric stretch centered at about 2923 cm−1, and (v) a CH3 antisymmetric stretch centered at about 2956 cm−1 to obtain an estimate of a CH2/CH3 ratio in kerogen, wherein the ratio is indicative of thermal maturity.
Methods exist to estimate the density of kerogen in bulk formation samples from IR spectroscopy, for example, as described in U.S. patent application Ser. No. 15/053,604, Methods for improving matrix density and porosity estimates in subsurface formations by Craddock, P. R., et al, the contents of which are hereby incorporated by reference in its entirety. These methods are also based on spectral deconvolution and curve fitting of measured IR spectral features between 2800 and 3000 cm−1, described generally above.
The spectral deconvolution and curve fitting techniques used depend on parameters such as the type of function used (e.g., Gaussian, Lorentzian, Voight, etc.), the number of curves to be solved, the peak centers of the curves, and the widths of the curves, not all of which are known.